To obtain a radiofrequency pattern that has a predefined outline, it is known practice to use a single feed associated with a system of single or double reflector(s) with shaped surface, that is to say a surface having a specific geometry defining, on the ground, a specific coverage area having a non-circular outline, for example a country or a group of countries. The optical path variations between the feed and the different points of the reflector make it possible to generate beams that have a phase and amplitude pattern corresponding to the characteristics of the desired radiofrequency pattern.
It is also possible, with only one reflector and by using two feeds placed as close as possible to the focus of the reflector, to obtain two different radiofrequency patterns making it possible to cover two different geographic coverage areas.
A reflector with shaped surface is generally manufactured by using a dedicated mould whose shape corresponds to a predetermined antenna coverage. For each new coverage, it is therefore necessary to remake a specific mould. For the mould to not be deformed at temperature during baking and make it possible to produce a reflector that has the specified profile, the moulds used are made of a material with low coefficient of thermal expansion CTE, for example a material comprising carbon fibres or a material consisting of a steel alloy such as Invar (registered trade mark) consisting of iron and nickel alloy fibres. The problem is that, for operation in the Ku band, it is necessary to achieve a very fine manufacturing accuracy involving a high number of iterations during which the profile of the mould is reworked and refined. Thus, for a reflector with a typical diameter of two meters the mould production time is between four to six months. So in order not to delay the development of a new satellite, the choice of the coverage area and the associated shaped surface of the reflector to be produced is therefore defined very early in the programme progress. The mould production time is therefore a very important constraint in the progress of a programme and, after the production of the mould has been launched, there is no longer any flexibility for subsequently redefining the coverage area to be produced.
To resolve this flexibility problem, a solution could consist in using a reconfigurable reflector by using a deformable reflecting surface. There are different types of deformable reflecting surfaces such as, for example, a flexible surface formed on a knitted fabric or a mesh fabric as described notably in the document FR 2 678 111 or a flexible surface using carbon fibres bound by a silicone or a flexible surface using a grid of orthogonal stiff wires whose edges are free, the grid being maintained and stressed to a predetermined shape only by control points.
It is also known, from the document EP 2 362 489, to produce a deformable reflecting membrane with high radiofrequency reflectivity comprising an alternating superposition of layers of conductive elastomer and at least two discontinuous reinforcing layers. This membrane allows for significant deformations in multiple directions in-plane and off-plane of the surface of the membrane, has a sufficient bending stiffness and a low thermal expansion coefficient allowing for a dimensional stability of the membrane over a temperature range compatible with a space application, and a good electrical homogeneity so as not to create passive intermodulation signals which could damage the receiving quality of the antenna. The membrane can be reconfigured by means of mechanical actuators.
However, the reconfigurable reflectors require the presence of a large number of mechanical actuators, fixed onto the bottom surface of the membrane at chosen positions, which push or pull on the membrane to deform it and give it the desired shape. These mechanical actuators often comprise rotary drive electric motors that can be coupled either with a ball joint, or with a nut system associated with a worm screw, the nut being fixed onto the membrane. The problem is that the presence of a large number of actuators significantly increases the cost of production of the reflector and its weight which is prejudicial in the case of a space application (particularly for a mission which does not require any reconfiguration).